LiFeS2 Battery with Mixed Salt Electrolyte

ABSTRACT

An nonaqueous electrolyte blend for improved low temperature performance in lithium-iron disulfide cells is contemplated. The electrolyte has at least two solutes, including lithium iodide, wherein the lithium iodide is present at a concentration of 0.26 to 0.45 moles per liter of solvent and wherein the overall concentration of solutes is between 0.40 and 0.75 moles of total solute per liter of solvents. The solvents include DIOX and DME provided at a ratio of DIOX:DME between 50:50 and 70:30, and the solvents must include at least 80 volume percent of ethers.

CROSS REFERENCE TO RELATED APPLICATIONS

The following application is a continuation of U.S. patent application Ser. No. 12/172,538, entitled “All-Temperature LiFeS₂ Battery with Ether and Low Concentration LiI Electrolyte” and filed on Jul. 14, 2008, currently pending, which is a continuation-in-part of U.S. patent application Ser. No. 11/204,694 entitled “Low Temperature Li/FeS₂ Battery” filed on Aug. 16, 2005, now abandoned, which is a continuation-in-part of U.S. Pat. No. 7,510,808, filed Aug. 27, 2004 and granted on Mar. 31, 2009.

BACKGROUND OF INVENTION

This invention relates to a primary nonaqueous electrolyte electrochemical battery cell, such as a lithium/iron disulfide cell, with good low temperature performance characteristics.

Batteries are used to provide power to many portable electronic devices. Common advantages of lithium batteries (those that contain metallic lithium or lithium alloy as the electrochemically active material of the negative electrode) include high energy density, good high rate and high power discharge performance, good performance over a broad temperature range, long shelf life and light weight. Lithium batteries are becoming increasingly popular as the battery of choice for new devices because of trends in those devices toward smaller size and higher power. The ability to use high power consumer devices in low temperature environments is also important. While lithium batteries can typically operate devices at lower temperatures than batteries with aqueous electrolytes, electrolyte systems that provide the best high power discharge characteristics, even after storage for long periods of time, do not always give the best performance at low temperatures.

One type of lithium battery, referred to below as a Li/FeS₂ battery, has iron disulfide as the electrochemically active material of the positive electrode. Li/FeS₂ batteries have used electrolyte systems with a wide variety of solutes and organic solvents. The salt/solvent combination is selected to provide sufficient electrolytic and electrical conductivity to meet the cell discharge requirements over the desired temperature range. While their polarity is relatively low compared to some other common solvents, ethers are often desirable because of their generally low viscosity, good wetting capability, good low temperature discharge performance and good high rate discharge performance. This is particularly true in Li/FeS₂ cells because the ethers are more stable than with higher voltage cathodes, so higher ether levels can be used. Among the ethers that have been used are 1,2-dimethoxyethane (DME) and 1,3-dioxolane (DIOX), which have been used together and in blends with other cosolvents. However, because of interactions among solvents, as well as with electrolyte solutes and electrodes, cell performance has been difficult to predict based on the properties of individual solvent and solute components.

A wide variety of solutes has been used in Li/FeS₂ cell electrolytes; lithium trifluoromethanesulfonate (also commonly referred to as lithium triflate or LiCF₃SO₃) is among them. An example of a Li/FeS₂ cell with a lithium triflate solute in a solvent blend comprising DIOX and DME is found in U.S. Pat. No. 4,952,330, which is hereby incorporated by reference. A solvent blend of 40 to 53 volume percent cyclic ether (e.g., DIOX), 32 to 40 volume percent linear aliphatic ether (e.g., DME) and 8 to 18 volume percent alkylene carbonate (e.g., propylene carbonate) is disclosed. However, such an electrolyte can result in poor cell discharge performance at high discharge rates.

Another example of a cell with an electrolyte containing lithium triflate dissolved in a solvent comprising DIOX and DME is found in U.S. Pat. No. 5,290,414, which is hereby incorporated by reference. A blend of from 1:99 to 45:55 DIOX:DME with an optional cosolvent (e.g., 0.2 weight percent 3,5-dimethylisoxazole (DMI)) is disclosed as a solvent. The disclosed cell had low impedance following storage at high temperature.

While electrolytes containing lithium triflate can provide fair cell electrical and discharge characteristics, such electrolytes have relatively low electrical conductivity, and lithium triflate has been relatively expensive. Lithium iodide (LiI) has been used as an alternative to lithium triflate to both reduce cost and improve cell electrical performance. U.S. Pat. No. 5,514,491, which is hereby incorporated by reference, discloses a cell with improved high rate discharge performance, even after storage at high temperature. LiI is the sole solute, and the electrolyte solvent comprises at least 97 volume percent ether (e.g., 20:80 to 30:70 by volume DIOX:DME, with 0.2 volume percent DMI as a cosolvent).

It has been discovered that when LiI is used as a solute in an electrolyte containing DME in the solvent, especially more than 40 volume percent, discharge capacity at low temperatures, such as −20° C. and below, can be very low. This is believed to be due to formation of a DME solvate that can precipitate from the electrolyte solution at low temperatures or otherwise degrade low temperature cell performance. Reducing the DME content in the solvent can prevent this problem, but some of the improvement in high rate and high power discharge performance realized with LiI as the solute is sacrificed. Copending U.S. patent application Ser. Nos. 10/928,943, filed Aug. 27, 2004, and 10/943,169, filed Sep. 16, 2004, which are hereby incorporated by reference, disclose cells in which this problem is solved by using an electrolyte solvent that either includes 1,2-dimethoxypropane (DMP) and less than 30 volume percent DME or includes 45 to 80 volume percent DME and 5 to 25 volume percent 3Me2Ox.

More recently it has been discovered that Li/FeS₂ cells with electrolytes that have a solvent with a high ether content and LiI as a solute (either the sole solute or in combination with lithium triflate) can, on high rate discharge at low temperatures, exhibit a rapid drop in voltage near the beginning of discharge. The voltage can drop so low that a device being powered by the cell will not operate. Eliminating LiI as a solute (e.g., by using lithium triflate as the sole solute) can solve this problem, but the operating voltage can then be too low on high rate and high power discharge at room temperature.

As a general rule, the electrolyte in any battery must be selected to provide sufficient electrolytic and electrical conductivity to meet the cell discharge requirements over the desired temperature range. As demonstrated by U.S. Pat. No. 4,129,691 to Broussely, increasing the solute (i.e., salt) concentration in a lithium battery electrolyte is expected to result in a corresponding increase in the conductivity and usefulness of that electrolyte (at least to a certain point), with higher conductivity presumed to be a desirable attribute. However, other limitations such as the solubility of the solute in specific solvents, the formation of an appropriate passivating layer on lithium-based electrodes and/or the compatibility of the solvent with the electrochemically active or other materials in the cell make the selection of an appropriate electrolyte system difficult. As a non-limiting example, U.S. Pat. No. 4,804,595 to Bakos describes how certain ethers are not miscible with solvents such as propylene carbonate. Additional electrolyte deficiencies and incompatibilities are well known and documented in this art, particularly as they relate to LiFeS₂ cells and lithium's reactivity with many liquids, solvents and common polymeric sealing materials.

Ethers are often desirable as lithium battery electrolyte solvents because of their generally low viscosity, good wetting capability, good low temperature discharge performance and good high rate discharge performance, although their polarity is relatively low compared to some other common solvents. Ethers are particularly useful in cells with pyrite because the cells tend to be more stable as compared to higher voltage cathode materials in ethers, where degradation of the electrode surface or unwanted reactions with the solvent(s) might occur (e.g., polymerization). Among the ethers that have been used in LiFeS₂ cells are 1,2-dimethoxyethane (DME) and 1,3-dioxolane (DIOX), whether used together as taught by U.S. Pat. No. 5,514,491 or 6,218,054 or European Patent 0 529 802 B1, all to Webber, or used in whole or in part as a blend of solvents as suggested by U.S. Pat. Nos. 7,316,868 to Gorkovenko (use of DIOX and 5-6 carbon 1,3-dialkoxyalkanes); 4,952,330 to Marple et al. (use of a set ratio of linear ethers, such as DME, cylic ethers, such as DIOX, and alkylene carbonates, such as propylene carbonate); 3,996,069 to Kronenberg (use of 3-methyl-2-oxazolidone and DIOX and/or DME); or U.S. Patent Publication No. 2008/0026296A1 to Bowden (use of sulfolane and DME). Other solvents not specifically containing DIOX or DME may also be possible, such as those disclosed in U.S. Pat. No. 5,229,227 to Webber (use of 3-methyl-2-oxazolidone with polyalkylyene glycol ethers such as diglyme). However, because of interactions among solvents, as well as the potential effects of solutes and/or electrode materials on those solvents, ideal electrolyte solvent blends and the resulting discharge performance of the cell are often difficult to predict without actually testing the proposed blend in a functioning electrochemical cell.

A wide variety of electrolyte solutes has been used for lithium-based cells, including lithium iodide (LiI), lithium trifluoromethanesulfonate (LiCF₃SO₃ or “lithium triflate”), lithium bistrifluoromethylsulfonyl imide (Li(CF₃SO₂)₂N or “lithium imide”), lithium perchlorate (LiClO₄), lithium hexafluoroarsenate (LiAsF₆) and others. While electrolytes containing lithium triflate can provide fair cell electrical and discharge characteristics, such electrolytes have relatively low electrical conductivity, lithium triflate is relatively expensive. Lithium iodide (LiI) has been used as an alternative to lithium triflate to both reduce cost and improve cell electrical performance, as discussed in the previously identified U.S. Pat. No. 5,514,491 to Webber. One particular brand of AA-sized FR06 batteries sold by Energizer Holdings Inc. currently includes a nonaqueous electrolyte with an approximate 0.75 molal concentration of LiI salt in a solvent mixture containing DIOX and DME.

Additives may be employed in the electrolyte to enhance certain aspects of a cell and/or its performance. For example, U.S. Pat. No. 5,691,083 to Bolster describes the use of a very low concentration of potassium salt additives to achieve a desired open circuit voltage in cells with a cathode material including FeS₂, MnO₂ or TiS₂. U.S. Publication No. 2008/0026290 to Jiang discloses the use of an aluminum additive to slow the development of a passivation film on the surface of the lithium electrode. In each of these examples, the benefit of the additive(s) selected must be balanced against any deleterious reactions or effects (in terms of discharge performance, safety and longevity of the battery).

Finally, as mentioned above, it is believed higher concentrations of solute(s) normally improve the conductivity of the electrolyte, although it is understood that there is ultimately a limit to this effect based upon certain variables, such as the viscosity of the solution (i.e., conductivity improves with concentration until the concentration makes the resulting solution too viscous, at which point conductivity drops off dramatically). Additionally, certain systems (typically in rechargeable lithium-sulfur battery systems where non-chalcogenic polysulfides are the preferred cathode material) utilize a “catholyte” where portions of the electrode itself dissolve into the electrolyte solution to provide ionic conductivity, such that the concentration of salt is an indicator as to the state of charge in the battery. In such systems, minimal to non-existent concentrations of lithium ions may be provided to a fully charged cell without compromising performance as taught by U.S. Pat. No. 7,189,477 to Mikhaylik. Ultimately, LiFeS₂ and other lithium electrochemical cells maintain an almost constant salt content in the electrolyte and do not exhibit this propensity to provide ions from the electrodes to the electrolyte. Therefore, catholyte systems have no relevance to LiFeS₂ systems in this regard, and more generally illustrate the pitfalls associated with blindly applying teachings from a given electrochemical system to another, dissimilar system.

In view of the above, an object of the present invention is to provide an economical nonaqueous electrolyte battery cell, particularly a primary Li/FeS₂ cell that does not exhibit a sharp voltage drop near the beginning of high rate and high power discharge at low temperature, while still providing reasonably good capacity on high rate and high power discharge at room temperature.

SUMMARY OF INVENTION

The above objects are met and the above disadvantages of the prior art are overcome by using an electrolyte having a solute comprising lithium iodide and one or more additional soluble salts.

An electrochemical cell using this electrolyte is also contemplated. The cell has an anode made of lithium or lithium alloy, such as lithium with less than 1% aluminum. The cathode includes iron disulfide coated onto a current collector, although any material with a potential versus lithium of less than or equal to 2.8 V may be considered. A separator is disposed between the two electrodes. The electrodes may be spirally wound into a jellyroll electrode assembly.

The electrolyte has a solvent containing at least 80 volume percent ethers, and the ethers include a 1,3-dioxolane based ether and a 1,2-dimethoxyethane based ether in a volume ratio greater than 45:55 and less than 85:15. The electrolyte also has a solute containing lithium iodide and one or more additional salts dissolved in the solvent, and the total solute concentration is from 0.40 to 2.00 moles per liter of solvent. When the electrolyte contains from 0.40 to 0.65 moles of solute per liter of solvent, the solute contains at least 35 mole percent lithium iodide, and when the electrolyte comprises from greater than 0.65 to 2.00 moles of solute per liter of solvent, the solute contains less than 35 mole percent lithium iodide. Preferably the additional salt(s) comprise lithium trifluoromethane sulfonate.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is an embodiment of a cylindrical cell with a lithium negative electrode, an iron disulfide positive electrode and a nonaqueous organic electrolyte.

FIG. 2 is a plot of capacity on the x-axis and voltage on the y-axis for nonaqueous electrolyte cells with different LiI concentrations in the electrolyte when discharged at a constant current of 1000 mA at −20° C.

FIG. 3 is a plot of capacity on the x-axis and voltage on the y-axis for nonaqueous electrolyte cells with different LiI concentrations in the electrolyte when discharged at a constant current of 1000 mA at −40° C.

FIGS. 4A, 4B, 4C and 4D show the results of LiI-based electrolytes with varying salt concentrations incorporated into FR6 cells that are discharged under varying drain rates and temperatures.

DETAILED DESCRIPTION OF INVENTION

Unless otherwise specified herein, all disclosed characteristics and ranges are as determined at room temperature (20-25° C.).

As used herein: 1) about means including normal variability due to sampling and measurement; 2) primary solute means the solute component that makes up more than 50 mole percent of the total amount of solute in an electrolyte; and 3) volumes of solvent components refer to the volumes of cosolvents that are mixed together to make the solvent for an electrolyte; volume ratios of cosolvents can be determined from the weight ratios of the cosolvents by dividing the relative weights of each of the cosolvents by their respective densities at 20° C. (e.g., 0.867 g/cm³ for DME, 1.176 g/cm³ for 3Me2Ox, 1.065 g/cm³ for DIOX and 0.984 g/cm³ for DMI).

Additionally, the terms listed below are defined and used throughout this disclosure as follows:

-   -   ambient (or room) temperature—between about 20° C. and about 25°         C.; unless otherwise stated, information is provided at ambient         temperature.     -   anode—the negative electrode; more specifically, within the         meaning of the invention, it consists essentially of lithium or         an alloy containing at least 90% lithium by weight as the         primary electrochemically active material.     -   cathode—the positive electrode; more specifically, within the         meaning of the invention, it comprises iron disulfide as the         primary electrochemically active material, along with one or         more rheological, polymeric and/or conductive additives, coated         onto a metallic current collector.     -   cell housing—the structure that physically encloses the         electrochemically active materials, safety devices and other         inert components which comprise a fully functioning battery;         typically consists of a container (formed in the shape of a cup,         also referred to as a “can”) and a closure (fitting over the         opening of the container, typically consists of venting and         sealing mechanisms for impeding electrolyte egress and         moisture/atmospheric ingress).     -   DIOX—a dioxolane-based solvent, typically 1,3-dioxolane     -   DME—a dimethoxyethane-based solvent, typically         1,2-dimethoxyethane     -   electrolyte—one or more solutes dissolved within one or more         liquid, organic solvents; but this definition does not include         electrochemical systems where the cathode is expected to         partially or completely dissolve in order to contribute ionic         conductivity to the cell (i.e., a “catholyte” such as those         utilized in lithium-sulfur batteries)     -   jellyroll (or spirally wound) electrode assembly—strips of anode         and cathode, along with an appropriate polymeric separator, are         combined into an assembly by winding along their lengths or         widths, e.g., around a mandrel or central core.     -   nominal—a value, specified by the manufacturer, that is         representative of what can be expected for that characteristic         or property.     -   salt—as part of the electrolyte, an ionizable compound,         typically including lithium or some other metal, dissolved in         one or more solutes.

Cell Design

The invention will be better understood with reference to FIG. 1, which shows a specific cell design that may be implemented. Cell 10 is an FR6 type cylindrical LiFeS₂ battery cell, although the invention should have equal applicability to FR03 or other cells. Cell 10 has a housing that includes a container in the form of a can 12 with a closed bottom and an open top end that is closed with a cell cover 14 and a gasket 16. The can 12 has a bead or reduced diameter step near the top end to support the gasket 16 and cover 14. The gasket 16 is compressed between the can 12 and the cover 14 to seal an anode or negative electrode 18, a cathode or positive electrode 20 and electrolyte within the cell 10.

The anode 18, cathode 20 and a separator 26 are spirally wound together into an electrode assembly. The cathode 20 has a metal current collector 22, which extends from the top end of the electrode assembly and is connected to the inner surface of the cover 14 with a contact spring 24. The anode 18 is electrically connected to the inner surface of the can 12 by a metal lead (or tab) 36. The lead 36 is fastened to the anode 18, extends from the bottom of the electrode assembly, is folded across the bottom and up along the side of the electrode assembly. The lead 36 makes pressure contact with the inner surface of the side wall of the can 12. After the electrode assembly is wound, it can be held together before insertion by tooling in the manufacturing process, or the outer end of material (e.g., separator or polymer film outer wrap 38) can be fastened down, by heat sealing, gluing or taping, for example.

An insulating cone 46 is located around the peripheral portion of the top of the electrode assembly to prevent the cathode current collector 22 from making contact with the can 12, and contact between the bottom edge of the cathode 20 and the bottom of the can 12 is prevented by the inward-folded extension of the separator 26 and an electrically insulating bottom disc 44 positioned in the bottom of the can 12.

Cell 10 has a separate positive terminal cover 40, which is held in place by the inwardly crimped top edge of the can 12 and the gasket 16 and has one or more vent apertures (not shown). The can 12 serves as the negative contact terminal. An insulating jacket, such as an adhesive label 48, can be applied to the side wall of the can 12.

Disposed between the peripheral flange of the terminal cover 40 and the cell cover 14 is a positive temperature coefficient (PTC) device 42 that substantially limits the flow of current under abusive electrical conditions. Cell 10 also includes a pressure relief vent. The cell cover 14 has an aperture comprising an inward projecting central vent well 28 with a vent hole 30 in the bottom of the well 28. The aperture is sealed by a vent ball 32 and a thin-walled thermoplastic bushing 34, which is compressed between the vertical wall of the vent well 28 and the periphery of the vent ball 32. When the cell internal pressure exceeds a predetermined level, the vent ball 32, or both the ball 32 and bushing 34, is forced out of the aperture to release pressurized gases from the cell 10. In other embodiments, the pressure relief vent can be an aperture closed by a rupture membrane, such as disclosed in U.S. Patent Application Publication No. 2005/0244706, herein fully incorporated by reference, or a relatively thin area such as a coined groove, that can tear or otherwise break, to form a vent aperture in a portion of the cell, such as a sealing plate or container wall.

The terminal portion of the electrode lead 36, disposed between the side of the electrode assembly and the side wall of the can, may have a shape prior to insertion of the electrode assembly into the can, preferably non-planar that enhances electrical contact with the side wall of the can and provides a spring-like force to bias the lead against the can side wall. During cell manufacture, the shaped terminal portion of the lead can be deformed, e.g., toward the side of the electrode assembly, to facilitate its insertion into the can, following which the terminal portion of the lead can spring partially back toward its initially non-planar shape, but remain at least partially compressed to apply a force to the inside surface of the side wall of the can, thereby making good physical and electrical contact with the can.

Electrolyte

A nonaqueous electrolyte, containing water only in very small quantities as a contaminant (e.g., no more than about 500 parts per million by weight, depending on the electrolyte salt being used), is deposited into the cell housing during manufacture. Because the electrolyte is the primary media for ionic transfer in a LiFeS₂ cell, selection of an appropriate solvent and solute combination is critical to optimizing the performance of the cell. Moreover, the solute and solvents selected for the electrolyte must possess appropriate miscibility and viscosity for the purposes of manufacture and use of the resulting cell, while still delivering appropriate discharge performance across the entire spectrum of temperatures potentially experienced by batteries (i.e., about −40° C. to 60° C.). Furthermore, the electrolyte must be non-reactive and non-volatile (or at least possess a low enough boiling point to be practically retained by conventional polymeric seals and closure mechanisms).

Miscibility and viscosity of the solvents and the electrolyte are key to the manufacturing and operational aspects of the battery. All solvents used in the blend must be completely miscible to insure a homogeneous solution. Similarly, in order to accommodate the requirements of high volume production, the solvents must possess a sufficiently low viscosity to flow and/or be dispensed quickly.

Additionally, the solvents and the electrolyte must possess a boiling point appropriate to the temperature range in which the battery will most likely be exposed and stored (i.e., −40° C. to 60° C.). More specifically, the solvent(s) must be sufficiently non-volatile to allow for safe storage and operation of the battery within this stated temperature range. Similarly, the solvents and the electrolyte must not react with the electrode materials in a manner that degrades the electrodes or adversely affects performance of the battery upon discharge. Suitable organic solvents that have been or may be used in LiFeS₂ cells have included one or more of the following: 1,3-dioxolane; 1,3-dioxolane based ethers (e.g., alkyl- and alkoxy-substituted DIOX, such as 2-methyl-1,3-dioxolane or 4-methyl-1,3-dioxolane, etc.); 1,2-dimethoxyethane; 1,2-dimethoxyethane-based ethers (e.g., diglyme, triglyme, tetraglyme, ethyl glyme, etc.); ethylene carbonate; propylene carbonate; 1,2-butylene carbonate; 2,3-butylene carbonate; vinylene carbonate; methyl formate; γ-butyrolactone; sulfolane; acetonitrile; N,N-dimethyl formamide: N,N-dimethylacetamide; N,N-dimethylpropyleneurea; 1,1,3,3-tetramethylurea; beta aminoenones; beta aminoketones; methyltetrahydrofurfuryl ether; diethyl ether; tetrahydrofuran (“THF”); 2-methyl tetrahydrofuran; 2-methoxytetrahydrofuran; 2,5-dimethoxytetrahydrofuran; 3,5-dimethylisoxazole (“DMI”); 1,2-dimethoxypropane (“DMP”); and 1,2-dimethoxypropane-based ethers (e.g., substituted DMP, etc.).

The electrolyte preferably comprises a solute dissolved in an organic solvent containing at least 80 volume percent ethers, including at least DIOX (e.g., 1,3-dioxolane and 1,3-dioxolane based ethers), and DME (e.g., 1,2-dimethoxyethane and 1,2-dimethoxyethane based ethers), with the DIOX and DME in a volume ratio greater than about 45:55 and less than about 85:15. Preferably the DIOX:DME volume ratio is no greater than about 75:25, more preferably no greater than about 70:30 and most preferably no greater than about 65:35. Preferably the DIOX:DME ratio is at least 50:50. When the ether content is too low, high rate discharge performance suffers, especially at low temperatures. When the DIOX:DME ratio is too low or too high, low temperature discharge capacity can be poor, and when the ratio is too high, discharge capacity in a digital still camera at room temperature can be poor. Preferably the total amount of DIOX and DME in the solvent is at least 80 volume percent, more preferably at least 90 volume percent. Examples of DIOX based ethers include alkyl- and alkoxy-substituted DIOX, such as 2-methyl-1,3-dioxolane and 4-methyl-1,3-dioxolane. Examples of DME based ethers include diglyme, triglyme, tetraglyme and ethyl glyme.

As previously noted, the solvent can also include additional cosolvents, examples of which include ethylene carbonate, propylene carbonate, 1,2-butylene carbonate, 2,3-butylene carbonate, vinylene carbonate, methyl formate, γ-butyrolactone, sulfolane, acetonitrile, 3,5-dimethylisoxazole, N,N-dimethyl formamide, N,N-dimethylacetamide, N,N-dimethylpropyleneurea, 1,1,3,3-tetramethylurea, beta aminoenones, beta aminoketones, and other ethers such as methyltetrahydrofurfuryl ether, diethyl ether, tetrahydrofuran, 2-methyl tetrahydrofuran, 2-methoxytetrahydrofuran, 2,5-dimethoxytetrahydrofuran, and 1,2-dimethoxypropane based compounds (1,2-dimethoxypropane and substituted 1,2-dimethoxypropane). DMI, DMP and 3Me2Ox are preferred cosolvents, particularly DMI. Because they can react with LiI, the solvent preferably contains a total of less than 5 volume percent, and more preferably, no dialkyl or cyclic carbonates.

Salts (interchangeably used along with the term “solute” herein) should be nearly or completely soluble with the selected solvent(s) and, as with the discussion of solvent characteristics above, without any degradation or adverse effects. Examples of typical salts used in LiFeS₂ cells include LiI (“lithium iodide”), LiCF₃SO₃ (“lithium triflate”), LiClO₄ (“lithium perchlorate”), Li(CF₃SO₂)₂N (“lithium imide”), Li(CF₃CF₂SO₂)₂N, LiBF₄ and Li(CF₃SO₂)₃C. Other potential candidates are lithium bis(oxalato)borate, lithium bromide, lithium hexafluorophosphate, potassium hexafluorophosphate and lithium hexafluoroarsenate. Two key aspects of salts are that they do not react appreciably with the housing, electrodes, sealing materials or solvents and that they do not degrade or precipitate out of the electrolyte under the typically expected conditions the battery will be exposed to (e.g., temperature, electrical load, etc.). It is possible to use more than one solute to maximize certain aspects of performance.

The solute includes LiI and one or more additional salts dissolved in the solvent. The total amount of solute in the electrolyte is between about 0.40 and about 2.00 moles per liter of solvent. Preferably the total solute concentration is at least 0.50 moles per liter of solvent. Preferably the total solute concentration is no greater than about 1.50 moles per liter of solvent, more preferably no greater than about 1.20 moles per liter of solvent. When the solute concentration is too high, the electrolyte solvent viscosity can be too high, leading to low operating voltages at low temperatures. When the concentration is too low, there are not enough lithium ions present to support high currents, and voltage is poor on high rate discharge at and below room temperature.

When the electrolyte contains from about 0.40 to about 0.65 moles of solute per liter of solvent, the solute contains at least about 35, preferably at least about 40, mole percent LiI. Within the range of 0.40 to 0.65 moles of solute per liter of solvent, the total solute concentration is more preferably from about 0.50 to 0.60 moles per liter of solvent. In a preferred embodiment with 0.40 to 0.65 moles of solute per liter of solvent, the mole ratio of LiI to the additional salt(s) is from about 60:40 to about 99:1, more preferably from about 60:40 to about 90:10, and most preferably from about 65:35 to about 75:25.

When the electrolyte contains from greater than about 0.65 to about 2.00 moles of solute per liter of solvent, the solute contains less than 35, preferably no more than about 30, mole percent LiI. Within the range of 0.65 to 2.00 moles of solute per liter of solvent, the total solute concentration is from about 0.70 to 1.20 moles per liter of solvent. In a preferred embodiment with 0.65 to 2.0 moles of solute per liter of solvent, the mole ratio of LiI to the additional salt(s) is from about 10:90 to about 30:70 and more preferably from about 10:90 to about 20:80. Preferably the LiI concentration is at least about 0.10 moles per liter of solvent. Preferably the LiI concentration is no greater than 0.20, moles per liter of solvent.

The additional soluble salt(s) can include one or a combination of salts that are stable in ether solvents. Lithium salts are preferred. Examples include LiCF₃SO₃, LiClO₄, Li(CF₃SO₂)₂N, Li(CF₃CF₂SO₂)₂N, Li(CF₃SO₂)₃C and lithium bis(oxalato)borate. LiCF₃SO₃ is a preferred lithium salt.

Other Cell Components

The cell container is often a metal can with a closed bottom such as the can in FIG. 1. The can material will depend in part of the active materials and electrolyte used in the cell. A common material type is steel. For example, the can may be made of steel, plated with nickel on at least the outside to protect the outside of the can from corrosion. The type of plating can be varied to provide varying degrees of corrosion resistance or to provide the desired appearance. The type of steel will depend in part on the manner in which the container is formed. For drawn cans the steel can be a diffusion annealed, low carbon, aluminum killed, SAE 1006 or equivalent steel, with a grain size of ASTM 9 to 11 and equiaxed to slightly elongated grain shape. Other steels, such as stainless steels, can be used to meet special needs. For example, when the can is in electrical contact with the cathode, a stainless steel may be used for improved resistance to corrosion by the cathode and electrolyte.

The cell cover can be metal. Nickel plated steel may be used, but a stainless steel is often desirable, especially when the cover is in electrical contact with the cathode. The complexity of the cover shape will also be a factor in material selection. The cell cover may have a simple shape, such as a thick, flat disk, or it may have a more complex shape, such as the cover shown in FIG. 1. When the cover has a complex shape like that in FIG. 1, a type 304 soft annealed stainless steel with ASTM 8-9 grain size may be used, to provide the desired corrosion resistance and ease of metal forming. Formed covers may also be plated, with nickel for example.

The terminal cover should have good resistance to corrosion by water in the ambient environment, good electrical conductivity and, when visible on consumer batteries, an attractive appearance. Terminal covers are often made from nickel plated cold rolled steel or steel that is nickel plated after the covers are formed. Where terminals are located over pressure relief vents, the terminal covers generally have one or more holes to facilitate cell venting.

The gasket is made from any suitable thermoplastic material that provides the desired sealing properties. Material selection is based in part on the electrolyte composition. Examples of suitable materials include polypropylene, polyphenylene sulfide, tetrafluoride-perfluoroalkyl vinylether copolymer, polybutylene terephthalate and combinations thereof. Preferred gasket materials include polypropylene (e.g., PRO-FAX® 6524 from Basell Polyolefins in Wilmington, Del., USA) and polyphenylene sulfide (e.g., XTEL™ XE3035 or XE5030 from Chevron Phillips in The Woodlands, Tex., USA). Small amounts of other polymers, reinforcing inorganic fillers and/or organic compounds may also be added to the base resin of the gasket.

The gasket may be coated with a sealant to provide the best seal. Ethylene propylene diene terpolymer (EPDM) is a suitable sealant material, but other suitable materials can be used.

If a ball vent is used, the vent bushing is made from a thermoplastic material that is resistant to cold flow at high temperatures (e.g., 75° C.). The thermoplastic material comprises a base resin such as ethylene-tetrafluoroethylene, polybutylene terephthlate, polyphenylene sulfide, polyphthalamide, ethylene-chlorotrifluoroethylene, chlorotrifluoroethylene, perfluoro-alkoxyalkane, fluorinated perfluoroethylene polypropylene and polyetherether ketone. Ethylene-tetrafluoroethylene copolymer (ETFE), polyphenylene sulfide (PPS), polybutylene terephthalate (PBT) and polyphthalamide are preferred. The resin can be modified by adding a thermal-stabilizing filler to provide a vent bushing with the desired sealing and venting characteristics at high temperatures. The bushing can be injection molded from the thermoplastic material. TEFZEL® HT2004 (ETFE resin with 25 weight percent chopped glass filler), polythlalamide (e.g., AMODEL® ET10011 NT, from Solvay Advanced Polymers, Houston, Tex.) and polyphenylene sulfide (e.g., e.g., XTEL™ XE3035 or XE5030 from Chevron Phillips in The Woodlands, Tex., USA) are preferred thermoplastic bushing materials.

The vent ball itself can be made from any suitable material that is stable in contact with the cell contents and provides the desired cell sealing and venting characteristic. Glasses or metals, such as stainless steel, can be used. In the event a foil vent is utilized in place of the vent ball assembly described above (e.g., pursuant to U.S. Patent Application Publication No. 2005/0244706), the above referenced materials may still be appropriately substituted.

Electrodes

The anode comprises a strip of lithium metal, sometimes referred to as lithium foil. The composition of the lithium can vary, though for battery grade lithium, the purity is always high. The lithium can be alloyed with other metals, such as aluminum, to provide the desired cell electrical performance or handling ease, although the amount of lithium in any alloy should nevertheless be maximized and alloys designed for high temperature application (i.e., above the melting point of pure lithium) are not contemplated. Appropriate battery grade lithium-aluminum foil, containing 0.5 weight percent aluminum, is available from Chemetall Foote Corp., Kings Mountain, N.C., USA.

Other anode materials may be possible, including sodium, potassium, zinc, magnesium and aluminum, either as co-anodes, alloying materials or distinct, singular anodes. Ultimately, the selection of an appropriate anode material will be influenced by the compatibility of that anode with LiI, the cathode and/or the ether(s) selected.

As in the cell in FIG. 1, a separate current collector (i.e., an electrically conductive member, such as a metal foil, on which the anode is welded or coated OR an electrically conductive strip running along the length of the anode) is not needed for the anode, since lithium has a high electrical conductivity. By not utilizing such a current collector, more space is available within the container for other components, such as active materials. Anode current collectors may be made of copper and/or other appropriate high conductivity metals so as long as they are stable when exposed to the other interior components of the cell (e.g., electrolyte).

The electrical connection must be maintained between each of the electrodes and the opposing terminals proximate to or integrated with the housing. An electrical lead 36 can be made from a thin metal strip connecting the anode or negative electrode to one of the cell terminals (the can in the case of the FR6 cell shown in FIG. 1). When the anode includes such a lead, it is oriented substantially along a longitudinal axis of the jellyroll electrode assembly and extends partially along a width of the anode. This may be accomplished embedding an end of the lead within a portion of the anode or by simply pressing a portion such as an end of the lead onto the surface of the lithium foil. The lithium or lithium alloy has adhesive properties and generally at least a slight, sufficient pressure or contact between the lead and electrode will weld the components together. The negative electrode may be provided with a lead prior to winding into a jellyroll configuration. The lead may also be connected via appropriate welds.

The metal strip comprising the lead 36 is often made from nickel or nickel plated steel with sufficiently low resistivity in order to allow sufficient transfer of electrical current through the lead and have minimal or no impact on service life of the cell, with a lead having less than 15 mΩ/cm and preferably less than 4.5mΩ/cm being ideal. A preferred material is 304 stainless steel. Examples of other suitable negative electrode lead materials include, but are not limited to, copper, copper alloys, for example copper alloy 7025 (a copper, nickel alloy comprising about 3% nickel, about 0.65% silicon, and about 0.15% magnesium, with the balance being copper and minor impurities); and copper alloy 110; and stainless steel. Materials should remain stable within the cell even after the nonaqueous electrolyte is added. Examples of metals generally to be avoided but can be present as impurities in relatively minor amounts are aluminum and zinc.

The cathode is in the form of a strip that comprises a current collector and a mixture that includes one or more electrochemically active materials, usually in particulate form. Iron disulfide (FeS₂) is a preferred active material although the invention is applicable to most cathode materials that are stable with LiI and have a potential vs. Li that is less than 2.8V, including CuO, CuO₂ and all oxides of bismuth (e.g., Bi₂O₃, etc.). Notably, MnO₂ is not suitable because it has a potential that is too high when compared to LiI.

In a LiFeS₂ cell, the cathode active material comprises greater than 50 weight percent FeS₂. The cathode can also contain one or more additional active materials mentioned above, depending on the desired cell electrical and discharge characteristics. More preferably the active material for a LiFeS₂ cell cathode comprises at least 95 weight percent FeS₂, and most preferably FeS₂ is the sole active cathode material. FeS₂ having a purity level of at least 95 weight percent is available from Washington Mills, North Grafton, Mass., USA; Chemetall GmbH, Vienna, Austria; and Kyanite Mining Corp., Dillwyn, Va., USA. A more comprehensive description of the cathode, its formulation and a manner of manufacturing the cathode is provided below.

The current collector may be disposed within or imbedded into the cathode surface, or the cathode mixture may be coated onto one or both sides of a thin metal strip. Aluminum is a commonly used material. The current collector may extend beyond the portion of the cathode containing the cathode mixture. This extending portion of the current collector can provide a convenient area for making contact with the electrical lead connected to the positive terminal. It is desirable to keep the volume of the extending portion of the current collector to a minimum to make as much of the internal volume of the cell available for active materials and electrolyte.

The cathode is electrically connected to the positive terminal of the cell. This may be accomplished with an electrical lead, often in the form of a thin metal strip or a spring, as shown in FIG. 1, although welded connections are also possible. The lead is often made from nickel plated stainless steel. Still another embodiment may utilize a connection similar to that disclosed in U.S. patent application Ser. No. 11/439,835, which should publish on or after Nov. 29, 2007, and/or U.S. patent application Ser. No. 11/787,436, which should publish on or after Oct. 16, 2008, both of which are commonly assigned to the assignee of this application and incorporated by reference herein. Notably, to the extent a cell design may utilize one of these alternative electrical connectors/current limiting devices, the use of a PTC may be avoided. In the event an optional current limiting device, such as a standard PTC, is utilized as a safety mechanism to prevent runaway discharge/heating of the cell, a suitable PTC is sold by Tyco Electronics in Menlo Park, Calif., USA. Other alternatives are also available.

Separator

The separator is a thin microporous membrane that is ion-permeable and electrically nonconductive. It is capable of holding at least some electrolyte within the pores of the separator. The separator is disposed between adjacent surfaces of the anode and cathode to electrically insulate the electrodes from each other. Portions of the separator may also insulate other components in electrical contact with the cell terminals to prevent internal short circuits. Edges of the separator often extend beyond the edges of at least one electrode to insure that the anode and cathode do not make electrical contact even if they are not perfectly aligned with each other. However, it is desirable to minimize the amount of separator extending beyond the electrodes.

To provide good high power discharge performance it is desirable that the separator have the characteristics (pores with a smallest dimension of at least 0.005 and a largest dimension of no more than 5 μm across, a porosity in the range of 30 to 70 percent, an area specific resistance of from 2 to 15 ohm-cm² and a tortuosity less than 2.5) disclosed in U.S. Pat. No. 5,290,414, issued Mar. 1, 1994, and hereby incorporated by reference.

Suitable separator materials should also be strong enough to withstand cell manufacturing processes as well as pressure that may be exerted on the separator during cell discharge without tears, splits, holes or other gaps developing that could result in an internal short circuit. To minimize the total separator volume in the cell, the separator should be as thin as possible, preferably less than 25 μm thick, and more preferably no more than 22 μm thick, such as 20 μm or 16 μm high tensile stress is desirable, preferably at least 800, more preferably at least 1000 kilograms of force per square centimeter (kgf/cm²). For an FR6 type cell the preferred tensile stress is at least 1500 kgf/cm² in the machine direction and at least 1200 kgf/cm² in the transverse direction, and for a FR03 type cell the preferred tensile strengths in the machine and transverse directions are 1300 and 1000 kgf/cm², respectively. Preferably the average dielectric breakdown voltage will be at least 2000 volts, more preferably at least 2200 volts and most preferably at least 2400 volts. The preferred maximum effective pore size is from 0.08 μm to 0.40 μm, more preferably no greater than 0.20 μm. Preferably the BET specific surface area will be no greater than 40 m²/g, more preferably at least 15 m²/g and most preferably at least 25 m²/g. Preferably the area specific resistance is no greater than 4.3 ohm-cm², more preferably no greater than 4.0 ohm-cm², and most preferably no greater than 3.5 ohm-cm². These properties are described in greater detail in U.S. Patent Publication No. 2005/0112462, which is hereby incorporated by reference.

Separator membranes for use in lithium batteries are often made of polypropylene, polyethylene or ultrahigh molecular weight polyethylene, with polyethylene being preferred. The separator can be a single layer of biaxially oriented microporous membrane, or two or more layers can be laminated together to provide the desired tensile strengths in orthogonal directions. A single layer is preferred to minimize the cost. Suitable single layer biaxially oriented polyethylene microporous separator is available from Tonen Chemical Corp., available from EXXON Mobile Chemical Co., Macedonia, N.Y., USA. Setela F20DHI grade separator has a 20 μm nominal thickness, and Setela 16MMS grade has a 16 μm nominal thickness. Suitable separators with similar properties are also available from Entek Membranes in Lebanon, Oreg., USA.

Cell Construction and Manufacture

The anode, cathode and separator strips are combined together in an electrode assembly. The electrode assembly may be a spirally wound design, such as that shown in FIG. 1, made by winding alternating strips of cathode, separator, anode and separator around a mandrel, which is extracted from the electrode assembly when winding is complete. At least one layer of separator and/or at least one layer of electrically insulating film (e.g., polypropylene) is generally wrapped around the outside of the electrode assembly. This serves a number of purposes: it helps hold the assembly together and may be used to adjust the width or diameter of the assembly to the desired dimension. The outermost end of the separator or other outer film layer may be held down with a piece of adhesive tape or by heat sealing. The anode can be the outermost electrode, as shown in FIG. 1, or the cathode can be the outermost electrode. Either electrode can be in electrical contact with the cell container, but internal short circuits between the outmost electrode and the side wall of the container can be avoided when the outermost electrode is the same electrode that is intended to be in electrical contact with the can.

The cell can be closed and sealed using any suitable process. Such processes may include, but are not limited to, crimping, redrawing, colleting and combinations thereof. For example, for the cell in FIG. 1, a bead is formed in the can after the electrodes and insulator cone are inserted, and the gasket and cover assembly (including the cell cover, contact spring and vent bushing) are placed in the open end of the can. The cell is supported at the bead while the gasket and cover assembly are pushed downward against the bead. The diameter of the top of the can above the bead is reduced with a segmented collet to hold the gasket and cover assembly in place in the cell. After electrolyte is dispensed into the cell through the apertures in the vent bushing and cover, a vent ball is inserted into the bushing to seal the aperture in the cell cover. A PTC device and a terminal cover are placed onto the cell over the cell cover, and the top edge of the can is bent inward with a crimping die to hold retain the gasket, cover assembly, PTC device and terminal cover and complete the sealing of the open end of the can by the gasket.

With respect to the cathode, the cathode is coated onto a metallic foil current collector, typically an aluminum foil with a thickness between 18 and 20 μm, as a mixture which contains a number of materials that must be carefully selected to balance the processability, conductivity and overall efficiency of the coating. This coating consists primarily of iron disulfide (and its impurities); a binder that is generally used to hold the particulate materials together and adhere the mixture to the current collector; one or more conductive materials such as metal, graphite and carbon black powders added to provide improved electrical conductivity to the mixture, although the amount of conductor depends upon the electrical conductivity of the active material and binder, the thickness of the mixture on the current collector and the current collector design; and various processing or rheological aids that are dependent upon the coating method, the solvent used and/or the mixing method itself.

The following are representative materials that may be utilized in the cathode mix formulation: pyrite (at least 95% pure); conductor (Pure Black 205-110 from Superior Graphite Chicago, Ill., and/or MX15 from Timcal Westlake, Ohio); and binder/processing aids (styrene-ethylene/butylenes-styrene (SEBS) block copolymer, such as g1651 from Kraton Polymers Houston, Tex., and Efka 6950 from Heerenveen, Netherlands). Small amounts of impurities may be naturally present in any of the aforementioned materials, although care should be taken to utilize the highest purity pyrite source available so as to maximize the amount of FeS₂ present within the cathode.

It is also desirable to use cathode materials with small particle sizes to minimize the risk of puncturing the separator. For example, FeS₂ is preferably sieved through a 230 mesh (62 μm) screen before use or the FeS₂ may be milled or processed as described in U.S. Patent Publication No. 2005/0233214, which is incorporated by reference herein. Other cathode mix components should be carefully selected with eye toward chemical compatibility/reactivity and to avoid similar particle-size-based mechanical failure issues.

The cathode mixture is applied to the foil collector using any number of suitable processes, such as three roll reverse, comma coating or slot die coating. The methods of coating described in U.S. patent application Ser. No. 11/493,314, which should publish on or after Jan. 31, 2008 and is incorporated by reference, could be used. One preferred method of making FeS₂ cathodes is to roll coat a slurry of active material mixture materials in a highly volatile organic solvent (e.g., trichloroethylene) onto both sides of a sheet of aluminum foil, dry the coating to remove the solvent, calender the coated foil to compact the coating, slit the coated foil to the desired width and cut strips of the slit cathode material to the desired length. The use of volatile solvents maximize the efficiency of recovering such solvents, although it is possible to utilize other solvents, including aqueous-based compositions, in order to roll coat the cathode mix described above.

After or concurrent with drying to remove any unwanted solvents, the resulting cathode strip is densified via calendering or the like to further compact the entire positive electrode. In light of the fact that this strip will then be spirally wound with separator and a similarly (but not necessarily identically) sized anode strip to form a jellyroll electrode assembly, this densification maximizes loading of electrochemical material in the jellyroll electrode assembly. However, the cathode cannot be over-densified as some internal cathode voids are need to allow for expansion of the iron disulfide during discharge and wetting of the iron disulfide by the organic electrolyte, as well as to avoid unwanted stretching and/or de-lamination of the coating.

Following assembly the cell can be predischarged, such as by discharging the cell by a small amount (e.g., removing a total of about 180 mAh of the cell capacity of an FR6 type cell) in one or more pulses.

The above description is particularly relevant to FR6 type cylindrical Li/FeS₂ cells with nonaqueous electrolytes and to pressure relief vents comprising a thermoplastic bushing and vent ball. However, the invention may also be adapted to other sizes and types of cells, such as button cells, pouch cells, non-cylindrical (e.g., prismatic) cells and cells with other pressure relief vent designs. Cells according to the invention can have spiral wound electrode assemblies, such as that shown in FIG. 1, or another electrode configuration, such as folded strips, stacked flat plates, bobbins and the like.

The present invention is useful for avoiding sharp voltage drops near the beginning of high rate and high power discharge at low temperatures. This phenomenon is different from a normal lowering of the cell discharge curve (e.g., voltage as a function of time on discharge) at low temperatures compared to room temperature, and electrolytes that improve one of these two conditions can actually worsen the other. The problem of sharp voltage drops in cells with electrolytes including LiI in a DIOX/DME solvent when discharged at high rates and very low temperatures as well as the features and advantages of the invention are illustrated in the following examples.

Example 1

FR6 type Li/FeS₂ cells similar to cell 10 in FIG. 1 were made to evaluate low temperature discharge performance on discharge at various constant current rates. The anode material was lithium metal alloyed with 0.5 weight percent aluminum (about 0.97 grams/cell average). The cathode was a strip of aluminum foil coated on both sides with cathode mixture (about 5.0 grams/cell) containing about 92 weight percent FeS₂, 1.4 weight percent acetylene black, 4 weight percent graphite, 2 weight percent binder, 0.3 weight percent micronized PTFE and 0.3 weight percent fumed silica. A 25 nm thick polypropylene separator was used. The average amount of electrolyte was about 1.6 grams per cell. The electrolyte contained a solvent blend of DIOX, DME and DMI in a ratio of 65:35:0.2 by volume LiI as the solute. Three lots of cells were made, each with a different concentration of LiI in the electrolyte (Lots 1, 2 and 3 with 0.3, 0.5 and 0.75 moles of LiI per liter of solvent, respectively). The cells were predischarged following assembly.

Cells from each of the lots were discharged continuously at a rate of 1000 mA at each of two temperatures: −20° C. and −40° C. Discharge curves, showing the capacity in Ah on the x-axis and cell voltage on the y-axis for representative cells from each lot, are shown in FIGS. 2 and 3. At −20° C. (FIG. 2) the cell capacity increases with increasing LiI concentration. The same is true at −40° C. (FIG. 3) for the lower LiI concentrations (0.3 and 0.5 moles per liter of solvent), but with 0.75 moles of LiI per liter of solvent, the cell voltage drops rapidly to less than 0.65 V, typically within the first several minutes on discharge, giving almost no useable capacity. In general, at low temperatures the lower the temperature the lower the operating voltage of the cell, resulting in reduced cell capacity, particularly to higher voltages. The sharp drop in voltage observed in Lot 3 is a different phenomenon.

Cells from Lot 3 were also discharged on a variety of different constant current rates ranging from 500 to 2000 mA and over a range of temperatures from −20 to −40° C. On the higher discharge rates and at the lower temperatures, the cell voltages dropped sharply, in some cases to a voltage well below a desired minimum (e.g., the minimum required to operate a device). While cell voltages were sometimes observed to recover as discharge continued, once a cell drops the minimum voltage required to operate a device, it would normally be considered fully discharged by a user, and the device turned off or the cell replaced before the cell voltage would recover to above the required minimum voltage. The occurrences of the sharp voltage drops observed and the corresponding minimum voltages are summarized in Table 1, in which an asterisk (*) indicates that no sharp voltage drop was observed, a voltage value indicates the minimum voltage observed, and “ - - - ” indicates no cells were tested. In general, sharp voltage drops were not observed at discharge rates of 1000 mA and below at −20° C., but at higher discharge rates sudden drops were observed, and the higher the discharge rate the lower the voltage dropped. At temperatures below −20° C., the lower the temperature, the lower the rate below which no sudden voltage drop is observed and the lower the sudden voltage drop for any given discharge rate.

TABLE 1 Discharge Temperature Rate (mA) −20° C. −25° C. −30° C. −35° C. −40° C. 500 * * * 1.27 1.11 600 * * * 1.19 1.02 700 * * * 1.03 0.61 800 * * 1.13 0.91 0.25 900 * * 0.99 0.85 0.09 1000 * 1.15 0.97 0.62 0.09 1300 1.09 — — — — 1500 0.98 — — — — 2000 0.84 — — — —

Example 2

FR6 cells were made using the same anode and cathode materials as in Example 1. However, the separator was 20 μm thick polyethylene (rather than 25 μm thick polypropylene), allowing increases in the amounts of lithium and cathode material to 0.99 and 5.17 grams, respectively. Eighteen lots of cells (Lots 4-21) were made using different electrolytes. As shown in Table 2, all electrolyte compositions had solvents consisting of DIOX and DME in varying ratios, as well as 0.2 volume percent DMI; and salts consisting of LiI and/or LiCF₃SO₃ (LiTFS) in varying ratios and varying total concentrations. Cells from each lot were discharged on 4 tests: (1) a digital still camera test (1.5 W×2 seconds, then 0.65 W×28 seconds, repeated 10 times per hour, 24 hours per day at room temperature to 1.1 volts), (2) a 1000 mA intermittent test (1000 mA 2 minutes on, then 5 minutes off, repeated continuously at −20° C. to 1.0 volt), (3) a 1250 mA intermittent test (1250 mA 6 minutes on, then 5 minutes off, repeated continuously at −30° C. to 0.773 volt), and (4) a 1250 mA continuous test (1250 mA continuous at −30° C. to 0.773 volt).

The results are summarized in Table 2; discharge capacities are indexed to Lot 21 (100×capacity/Lot 21 capacity); the relative capacity of Lot 21 is 100 on each test. An asterisk (*) indicates those lots in which sudden voltage drops to below the end voltage occurred. The results show that with a high DIOX:DME ratio of 85:15, capacity on the DSC test at room temperature was less than Lot 21, particularly when a mixed LiI/LiTFS salt is used. Capacity on the DSC test at room temperature was better than Lot 21 when a low DIOX:DME ratio of 45:55 was used, but low temperature performance was poor.

TABLE 2 DSC to 1000 mA 1250 mA 1250 mA 1.1 V inter. to inter. to cont. to LiI/ Total (room 1.0 V 0.773 V 0.773 V Lot No. DIOX/DME LiTFS Salt LiI temp.) (−20 C.) (−30° C.) (−30° C.) 11 85/15 100/0  1 1 97 <1 <1 * 7 85/15 100/0  0.75 0.75 84 1 <1 * 13 85/15 85/15 0.875 0.744 90 2 <1 * 9 85/15 70/30 1 0.7 89 2 <1 * 5 85/15 70/30 0.75 0.525 73 66 <1 * 15 65/35 100/0  0.875 0.875 106 103 <1 22 20 65/35 100/0  0.75 0.75 101 95 61 98 17 65/35 85/15 1 0.85 107 100 <1 <1 18 65/35 85/15 0.875 0.744 105 98 106 50 19 65/35 85/15 0.875 0.744 104 103 2 64 16 65/35 85/15 0.75 0.638 100 97 112 102 14 65/35 70/30 0.875 0.613 102 98 105 99 10 45/55 100/0  1 1 110 12 3 * 6 45/55 100/0  0.75 0.75 108 31 6 * 12 45/55 85/15 0.875 0.744 107 31 7 * 8 45/55 70/30 1 0.7 106 40 9 * 4 45/55 70/30 0.75 0.525 101 78 19 * 21 (index/ 65/35 100/0  0.75 0.75 100 100 100 100 control)

Example 3

FR6 cells similar to those in Example 2 were made using various electrolytes. All electrolytes had solvents consisting of DIOX and DME, in varying ratios, as well as DMI; the ratio of the combination of DIOX and DME to DMI was 99.8:0.2 by volume. All electrolytes had solutes consisting of LiI in varying concentrations, ranging from 0.5 to 1.5 moles per liter of solvent, as shown in Table 3.

Cells from each lot were discharged on each of three tests: (1) a DSC test similar to that described in Example 2, except the end voltage was 1.05 rather than 1.1 V, (2) a 1000 mA continuous test to 1.0 V at room temperature, and (3) a 1000 mA continuous test to 1.0 V at −20° C.). The average capacities, indexed to Lot 23 (made like Lot 21 in Example 2), are summarized in Table 3. In general, the higher the LiI concentration, the higher the high rate discharge capacity at room temperature, but with 1.5 moles of LiI per liter of solvent, sudden voltage drops resulted in little capacity on 1000 mA continuous discharge at −20° C.

TABLE 3 1000 mA DSC to cont. to 1000 mA LiI 1.05 V 1.0 V cont. to DIOX/DME (mol/l (room (room 1.0 V Lot No. (vol.) solvent) temp.) temp.) (−20° C.) 22 65/35 0.5 92 90 99 23 65/35 0.75 100 100 100 24 65/35 1 104 103 84 25 65/35 1.25 104 108 109 26 65/35 1.5 107 107 1

Example 4

FR6 cells similar to those in Example 2 were made using various electrolytes. All electrolytes had solvents consisting of DIOX and DME, in varying ratios, as well as 0.2 volume percent DMI; the ratio of the combination of DIOX and DME to DMI was 99.8:0.2 by volume. All electrolytes had solutes consisting of LiI, LiTFS or a mixture thereof. The DIOX:DME ratio, total solute concentration and LiI concentration for each lot are included in Table 4.

Cells from each lot were tested on a composite discharge test at room temperature. On this test, each cell was first discharged continuously to 1.0 V on a series of constant current segments (2000 mA, 1500 mA, 1000 mA, 750 mA, 500 mA, 400 mA, 300 mA, 200 mA, 100 mA and 20 mA), with 2 hours rest between successive discharge segments. The cumulative capacities after the 2000, 1000, 200 and 20 mA segments of the test, indexed to Lot 42, are summarized in Table 4. Overall, the best performance was with Lot 42, those lots with higher LiI/LiCF₃SO₃ ratios and/or higher LiI concentrations performed better on the high rate discharge segments of the test, and those lots with only LiCF₃SO₃ as a solute performed very poorly on high rate discharge.

TABLE 4 Total LiI/ Salt LiI DIOX/DME LiTFS (mol/l (mol/l Lot No. (vol.) (mol.) solvent) solvent) 2000 mA 1000 mA 200 mA 20 mA 27 60/40 70/30 0.75 0.525 90 98 98 98 28 60/40 35/65 0.65 0.228 62 73 96 98 29 70/30 35/65 0.75 0.263 67 78 97 98 30 50/50 70/30 0.65 0.455 84 95 97 98 31 50/50 35/65 0.75 0.263 81 93 96 96 32 70/30 35/65 0.55 0.193 11 54 95 98 33 50/50  0/100 0.65 0.000 1 51 96 100 34 60/40 35/65 0.65 0.228 67 82 95 96 35 50/50 35/65 0.55 0.193 52 70 95 97 36 60/40  0/100 0.55 0.000 <1 26 92 99 37 60/40 70/30 0.55 0.385 66 83 95 96 38 70/30  0/100 0.65 0.000 <1 38 93 98 39 70/30 70/30 0.65 0.455 80 92 96 97 40 60/40 35/65 0.65 0.228 47 78 96 97 41 60/40  0/100 0.75 0.000 1 54 97 101 42 65/35 100/0  0.75 0.750 100 100 100 100

Cells from each lot were also tested on each of three tests: (1) the DSC test described in Example 3, (2) a 1000 mA continuous test to 1.0 Vat −20° C., and (3) the 1250 mA continuous test described in Example 2. The results, summarized in Table 5, are indexed to Lot 42 (made like Lots 21 and 23 above), except for the 1250 mA continuous test, on which the cells from Lot 42 gave essentially no capacity due to their rapid voltage drop to less than 0.773 V; for this test the results are shown in minutes. In general, the relationship among lots on the DSC test was similar to the relationships among lots on the high rate portions of the composite test summarized in Table 4. However, on the 1250 mA continuous test at −30° C., the lots that performed best at room temperature (Lots 42 and 27) exhibited sudden voltage drops, resulting in essentially no capacity.

TABLE 5 1250 mA DSC to 1000 mA cont. to 1.05 V cont. to 0.773 V LiI/ Total (room 1.0 V (−30° C.) Lot No. DIOX/DME LiTFS Salt LiI temp.) (−20 C.) MIN. 39 70/30 70/30 0.65 0.455 87 116 19 29 70/30 35/65 0.75 0.263 72 65 44 32 70/30 35/65 0.55 0.193 44 16 15 38 70/30  0/100 0.65 0 <1 1 2 27 60/40 70/30 0.75 0.525 94 99 0 37 60/40 70/30 0.55 0.385 85 79 42 28 60/40 35/65 0.65 0.228 75 71 38 34 60/40 35/65 0.65 0.228 73 75 74 40 60/40 35/65 0.65 0.228 76 74 41 41 60/40  0/100 0.75 0 31 59 56 36 60/40  0/100 0.55 0 <1 1 2 30 50/50 70/30 0.65 0.455 93 120 28 31 50/50 35/65 0.75 0.263 84 119 60 35 50/50 35/65 0.55 0.193 63 68 42 33 50/50  0/100 0.65 0 2 19 37 42 (index/ 65/35 100/0  0.75 0.75 100 100 0 control)

Example 5

Statistical analyses of discharge test data for cells from Examples 2 and 4 were done using DESIGN EXPERT® software from Stat-Ease Inc., Minneapolis, Minn., USA, to predict the best electrolyte formulation parameters for optimizing capacity on the DSC test at room temperature (to 1.1 V for Example 2, 1.05 V for Example 4), 2000 mA continuous discharge at room temperature, 1000 mA continuous discharge at −20° C. and 1250 mA continuous discharge at −30° C. The results (best electrolyte parameters and predicted capacities) are summarized in Table 6; predicted capacities are indexed to cells with an electrolyte having a solvent consisting of DIOX, DME and DMI in a volume ratio of 65:35:0.2 and a solute consisting of 0.75 moles of LiI per liter of solvent. The asterisks (*) indicate no predicted rapid voltage drop below 0.773 V on 1250 mA discharge at −30° C.

TABLE 6 Electrolyte Relative Capacity Parameter or Best DSC Best 2000 mA Best 1000 mA Best 1250 mA Discharge at room temp. at room temp. at −20° C. at −30° C. Test Ex. 2 Ex. 4 Ex. 2 Ex. 4 Ex. 2 Ex. 4 Ex. 2 Ex. 4 Ex. 4 DIOX/DME 53.6/46.4 57.4/42.6 52.6/47.4 61.3/38.7 64.7/35.3 52.5/47.5 64.9/35.1 50.2/49.8 51.6/48.4 (vol.) Total solute 0.937 0.726 0.992 0.748 0.807 0.727 0.751 0.554 0.750 (moles/l) LiI/LiTFS 100/0  64.3/35.7 97.1/2.9  69.7/30.3 78.8/21.2 67.0/33.0 70.4/29.6 68.3/31.7  0.2/99.8 (moles) LiI 0.937 0.466 0.964 0.522 0.635 0.487 0.529 0.378 0.001 (moles/l) LiTFS 0.000 0.259 0.029 0.226 0.171 0.240 0.222 0.176 0.748 (moles/l) 2000 mA 116 69 119 75 104 71 99 56 18 to 1.0 V, room temp. DSC test, 111 109 111 111 100 108 94 85 25 room temp. 1000 mA 77 109 70 111 117 121 146 95 63 to 1.0 V, −20° C. 1250 mA 13 * 9 * 38 * 46 * * to 1.0 V, −30° C.

The surface response chart generated by the statistical analysis software was used to select suitable ranges for electrolyte composition parameters disclosed above expected to provide usable capacity on 1250 mA discharge at −30° C., good capacity on 1000 mA discharge at −20° C. and minimal loss in high rate capacity, compared to cells with an electrolyte containing 0.75 moles of LiI per liter of solvent consisting of DIOX, DME and DMI in a volume ratio of 65:35:0.2.

Example A

A series of LiFeS₂ cells were constructed according to the design depicted in FIG. 1 and the preferred materials described above. Polypropylene gaskets were used. The solvent blend was 65 vol. % DIOX, 35 vol. % DME and 0.2 vol. % DMI. Only the concentration and composition of solute provided to these cells was varied, as described in the tables and further examples below.

Four different solutes were studied: lithium iodide, lithium imide, lithium perchlorate and lithium triflate. Cell lots for each solute were formulated at four different concentration levels using the criteria set forth above. These cells were then discharged at a variety of different temperatures, as set forth in Table A below. The cells using LiClO₄ were not tested at 21° C., but typically such cells give comparable performance to equivalent electrolytes using LiI at that temperature.

TABLE A 1A Continuous Drain at Varying Temperatures and Concentrations. (mAh to 0.9 V cut) Solute Molality −40° C. −20° C. 21° C. Lithium 0.50 1,172 2,056 2,716 Iodide 0.75 2 2,133 2,872 1.00 2 2,055 2,932 1.25 1 6 2,903 Lithium 0.67 333 1,030 1,576 Triflate 0.80 494 1,280 1,890 1.00 131 1,439 2,150 1.33 231 1,734 2,539 1.67 12 1,426 2,652 Lithium 0.50 41 2,428 Perchlorate 0.60 37 2,409 0.75 1 2,314 1.00 0 2,213 1.25 0 2,656 Lithium 0.50 1,214 1,917 2,929 Imide 0.75 878 2,475 3,053 1.00 1,049 2,433 3,020 1.25 849 2,232 3,030

The data above demonstrates that lithium perchlorate and lithium triflate do not provide adequate service across the entire spectrum of temperatures. Even within the range of lithium iodide concentrations studied, only the lowest molality electrolyte demonstrated consistent performance without significant degradation of performance at ambient temperatures. While lithium imide appears to provide acceptable service across the range, it does not display any definitive trends with respect to the concentration of solute provided (excepting, perhaps, at room temperature, where there is a slight advantage to providing higher concentrations). The imide's cost also places it a disadvantage in comparison to the other solutes.

Example B

Given the results of Example A, lithium imide electrolytes were compared against lithium iodide to test the electrolyte's ability to withstand long term storage and storage at elevated temperatures. It is believed storage at elevated temperatures accelerates and simulates the effects of long term storage at room temperature. Notably, cells must display only a minimal rise in its impedance and OCV, while maintaining acceptable flash amperage, in order to provide adequate long term storage.

Cells were made according to Example A above, but focusing on lithium imide electrolyte formulations, which are still benchmarked against lithium iodide, whose viability for long term storage is already well known/established (FR6 cells with lithium iodide have a shelf life of at least 10-15 years). The cells of this Example B were then stored at 60° C. for 6 weeks, while being periodically checked for OCV, impedance and flash amperage, as set forth in Table B1 below.

TABLE B1 Comparison of Long Term Storage Traits at 60° C. No. Weeks OCV 10 Hz Flash Electrolyte Stored (volts) Impedance Amps 0.75 m Iodide 0 1.771 0.122 15.6 (control) 1 1.827 0.139 16.8 2 1.836 0.155 16.1 4 1.854 0.189 14.9 6 1.857 0.221 13.4 0.50 m Imide 0 1.784 0.174 15.1 1 1.850 0.169 15.3 2 1.856 0.217 14.1 4 1.871 0.418 10.8 6 1.878 0.580 9.6 0.75 m Imide 0 1.791 0.150 15.7 1 1.857 0.143 17.0 2 1.860 0.165 16.3 4 1.871 0.245 13.6 6 1.873 0.308 11.9 1.00 m Imide 0 1.796 0.183 16.0 1 1.861 0.209 16.2 2 1.863 0.260 15.5 4 1.875 0.416 12.6 6 1.875 0.527 11.4 1.25 m Imide 0 1.797 0.160 15.8 1 1.861 0.184 16.4 2 1.863 0.223 15.9 4 1.875 0.332 13.4 6 1.876 0.433 12.3

While all lots studied displayed some negative effects after long term storage, virtually all of the imide lots, irrespective of concentration, displayed significantly worse performance as compared to the iodide. Therefore, lithium imide may not provide enough long term storage capability to meet consumer expectations.

In comparison, long term storage data for low molality LiI electrolytes are shown in Table B2 below. Note that these figures were generated in a separate experiment, although comparative context is provided by a “control” lot of 0.75 m LiI electrolyte, although restraints on resources prevented conducting this experiment beyond 4 weeks of storage time. On the balance, the control lot performed slightly worse in Table B2 as compared to Table B1, and the 0.35 m Iodide lot's performance may have been similarly degraded. More significantly, the flash amps for the low molality lot did not appear to degrade with storage time, and the increase in impedance was significantly less as compared to the increase over a similar period of time for all imide lots from Example B.

TABLE B2 Long Term Storage Traits for Low Molality LiI at 60° C. No. Weeks OCV 10 Hz Flash Electrolyte Stored (volts) Impedance Amps 0.75 m Iodide 0 1.766 0.035 15.5 (control) 1 1.841 0.082 15.5 2 1.842 0.106 15.3 4 1.851 0.115 13.0 0.35 m Iodide 0 1.765 0.080 10.3 1 1.858 0.141 9.9 2 1.863 0.152 10.9 4 1.870 0.150 10.2

Example C

A further study of low concentrations of lithium iodide was conducted across the range of temperatures to confirm both the benefits achieved at low temperature and the level of service provided at ambient temperatures. Accordingly, cells were made according to Example A above at 0.40 molal, 0.50 molal and 0.75 molal LiI concentrations, with the results shown for various temperatures in FIGS. 4A, 4B, 4C and 4D. Note that “DSC” in FIG. 4D refers to the ANSI digital still camera test, an intermittent pulsing drain rate test. The “Signature Test” in FIGS. 4A, 4B and 4C refers to a sequential continuous drain rate test where the cell is progressively discharged to a predetermined cutoff voltage, then retested at a lower drain rate after a rest period. In each of FIGS. 4A, 4B, 4C and 4D, the “control” electrolyte is a 0.75 m LiI formulation and virtually all electrolytes in these Figures used a solvent blend identical to the blend of Example 1.

Note that the room temperature (21° C.) and −20° C. data in FIGS. 4A and 4B is roughly comparable for all electrolytes, although there is slightly better service from the higher concentration electrolytes at higher drain rates (i.e., 1000 mA continuous and DSC). However, this slightly better service at higher temperatures pales in comparison to the vastly superior performance of the low molality electrolyte on all drain rates tested at −40° C. as shown in FIG. 4C. FIG. 4D merely illustrates the comparative performance of the electrolytes on the DSC test across a range of temperatures.

Example D

Another lot of cells were constructed with identical features but also including an electrode assembly designed to deliver more capacity, irrespective of electrolyte blend. Here the LiI electrolytes were provided at 0.75 m, 0.5 m and 0.35 m. These cells were drained at a continuous 200 mA rate at two separate temperatures. The 0.75 m cells provided 2,933 mAh to a 1V cut at 21° C. but only 203 mAh to a 1V cut at −40° C. The 0.5 m cells yielded 2,917 mAh and 617 mAh and the 0.35 m cells 2,847 mAh and 1,343 mAh, respectively speaking. As such, the electrolyte according to the invention provided over 97% of the capacity demonstrated of the prior art at ambient temperature, while delivering a 300% to 600% improvement over the prior art at low temperature, even when that prior art electrolyte was used in conjunction with an electrode specifically made for low temperature performance.

When all of the data is considered in total, the lower concentration electrolytes perform at nearly the same level as the prior art formulation; however, quite unexpectedly, at low temperature and higher drain rates, these low concentration formulations perform well beyond the expected service levels.

Example E

Low concentration (0.35 m) lithium lithium iodide electrolytes according to the invention were incorporated into the cell design and construction discussed in Example A, along with control electrolytes (0.75 m). The undischarged cells were then monitored for temperature increase during impact and crush tests—extremely important metrics for safety in the LiFeS₂ battery field. 10 batteries from each lot were monitored for temperature rise on impact, with the lower concentration electrolyte cells averaging a maximum high of 57.2° C. as compared to 72.6° C. for the controls. 20 batteries from each lot were monitored for high temperature on the crush test, with the average low concentration cells at 26.5° C. and the control lot at 38.9° C.

Features of the invention and its advantages will be further appreciated by those practicing the invention, particularly with reference to the Examples, Figures, Tables and other information provided herein and any patent references above necessary to better understand the invention are incorporated herein to that extent. In the same manner, it will be understood by those who practice the invention and those skilled in the art that various modifications and improvements may be made to the invention without departing from the spirit of the disclosed concepts. The scope of protection afforded is to be determined by the claims and by the breadth of interpretation allowed by law. 

1. An FR6 or FR03 battery comprising: a spirally wound electrode assembly including an anode consisting essentially of lithium or a lithium alloy and a cathode comprising iron disulfide coated onto a metallic foil; and an nonaqueous electrolyte consisting essentially of at least two solutes, including lithium iodide, dissolved in a blend of solvents having at least 80 volume percent ethers; wherein the ethers include 1,3-dioxolane and 1,2-dimethoxyethane provided in a volume ratio of 1,3-dioxolane:1,2-dimethoxyethane between 50:50 and 70:30; wherein the solutes are provided at 0.40 and 0.75 moles of solutes per liter of solvents; and wherein the lithium iodide is provided at between 0.26 and 0.45 moles of lithium iodide per liter of solvents.
 2. The battery of claim 1, wherein the battery does not experience a sudden voltage dropoff when tested at a temperature of less than −30° C. and at a continuous drain of at least 1.25 A.
 3. The battery of claim 1, wherein the solutes consist essentially of lithium iodide and at least one selected from the group consisting of: LiCF₃SO₃, LiClO₄, Li(CF₃SO₂)₂N, Li(CF₃CF₂SO₂)₂N, Li(CF₃SO₂)₃C and lithium bis(oxalato)borate.
 4. The battery of claim 1, wherein the solutes consist essentially of lithium iodide and at least one selected from the group consisting of: LiCF₃SO₃, LiClO₄, Li(CF₃SO₂)₂N, Li(CF₃CF₂SO₂)₂N, LiBF₄, Li(CF₃SO₂)₃C, lithium bis(oxalato)borate, lithium bromide, lithium hexafluorophosphate, potassium hexafluorophosphate and lithium hexafluoroarsenate.
 5. The battery of claim 1, wherein the solvents consistent essentially of 1,3-dioxolane, 1,2-dimethoxyethane and one or more optional cosolvents.
 6. The battery of claim 5, wherein the one or more optional cosolvents is present and selected from the group consisting of: 3,5-dimethylisoxazole, 1,2-dimethoxypropane, 3-methyl-2-oxazolidinone, and beta aminoenones.
 7. The battery of claim 5, wherein the one or more optional cosolvents is present and selected from the group consisting of: 1,3-dioxolane based ethers, 1,2-dimethoxyethane-based ethers, ethylene carbonate, propylene carbonate, 1,2-butylene carbonate, 2,3-butylene carbonate, vinylene carbonate, methyl formate, γ-butyrolactone, sulfolane, acetonitrile, N,N-dimethyl formamide, N,N-dimethylacetamide, N,N-dimethylpropyleneurea, 1,1,3,3-tetramethylurea, beta aminoenones, beta aminoketones, methyltetrahydrofurfuryl ether, diethyl ether, tetrahydrofuran, 2-methyl tetrahydrofuran, 2-methoxytetrahydrofuran, 2,5-dimethoxytetrahydrofuran, 3,5-dimethylisoxazole, 1,2-dimethoxypropane, and 1,2-dimethoxypropane-based ethers.
 8. The battery of claim 1, wherein the solutes are provided at between 0.40 and 0.65 moles of solutes per liter of solvents. 